Previous investigations have demonstrated an induction of relaxin secretion in monkeys administered hCG during the luteal phase of the menstrual cycle. Recent studies utilizing hCG at levels that mimic early pregnancy indicate that the pattern of hCG induced relaxin secretion is dependent on the age of the corpus luteum at the time of initial hormonal administration. During early, middle and late luteal phases (days 5, 8 and 12 following LH surge respectively), hC administration resulted in detectable levels of relaxin in 9.0 days, 6.6 days and 4.7 days, respectively. Serum progesterone decreased to minimal levels (Less than 2 ng/ml) prior to relaxin secretion. Relaxin secretion was sustained only in those animals receiving hCG at midluteal phase, the time of expected implantation in a fertile cycle. Thus, these observations may support a role for relaxin during early pregnancy.